1. Field of the Invention
This invention relates to a process for the controlled polymerization of stereospecific alpha-olefins having a preselected level of xylene solubles, more specifically, a process for controlling xylene solubles in polypropylene to the minimum level by use of the molar ratio of co-catalyst to external electron donor (selectivity control agent).
2. Description of the Prior Art
Polypropylene manufacturing processes typically involve the polymerization of propylene monomer with an organometallic catalyst of the Ziegler-Natta type. The Ziegler-Natta type catalyst polymerizes the propylene monomer to produce predominantly solid crystalline polypropylene. Many desirable product properties, such as strength and durability, depend on the crystallinity of the polypropylene which in turn is dependent on the stereospecific arrangement of methyl groups on the polymer backbone. One form of crystalline polypropylene is isotactic polypropylene in which the methyl groups are aligned on the same side of the polymer chain as opposed to atactic polypropylene in which the methyl groups are randomly positioned.
The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or all below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR spectroscopy. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
This crystallinity distinguishes isotactic polymers from an amorphous or atactic polymer which is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. While it is possible for a catalyst to produce both amorphous and crystalline, it is desirable for a catalyst to produce predominantly crystalline polymer with very little atactic polymer.
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst; a co-catalyst, usually an organoaluminum compound; and an external electron donor compound or selectivity control agent, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos.: 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference. These are just a few of the scores of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene.
A Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl, typically an organoaluminum compound, as a co-catalyst. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum co-catalyst.
The development of these polymerization catalysts has proceeded seemingly in generations of catalysts. The catalysts disclosed in the patents referenced above are considered by most to be third generation catalysts. With each new generation of catalysts, the catalyst properties have improved, particularly the efficiencies of the catalysts, as expressed in kilograms of polymer product per gram of catalyst in two hours.
In addition to the improved catalysts, improved activation methods have also lead to increases in the catalyst efficiency. A most recent discovery includes a process for pre-polymerizing the catalyst just prior to introducing the catalyst into the reaction zone. This process is disclosed in U.S. Pat. No. 4,767,735 the disclosure of which is hereby incorporated by reference.
It is generally possible to control catalyst productivity and product isotacticity within limits by adjusting the molar feed ratio of co-catalyst to external electron donor. Increasing the amount of external electron donor decreases the xylene solubles but may reduce activity and hence catalyst productivity. The stereoselectivity can be measured by the Isotactic Index (II) or the Xylene Solubles (XS) of the polypropylene product.
Selectivity to isotactic polypropylene is typically determined under the XS test by measuring the amount of polypropylene materials which are xylene soluble. The xylene-solubles were measured by dissolving polymer in hot xylene, cooling the solution to 0.degree. C. and precipitating out the crystalline material. The xylene solubles are the wt. % of the polymer that was soluble in the cold xylene.
The Isotactic Index (II), on the other hand, measures the amount of polypropylene material insoluble in n-heptane. Although the two tests, XS and II, are generally run using different solvents, they generate results which are predictably related since one test (XS) measures insolubility and the other (II) measures solubility. Both XS and II can be measured using known laboratory sampling techniques.
It would be advantageous to determine the optimum molar ratio of co-catalyst to external electron donor to minimize the amount of xylene solubles in polypropylene.